1. Field of the Invention
The present invention relates to an apparatus for the adjustment of the tension or gripping force to a rotating gripping device of a machine tool having multiple gripping elements, operated by a tension producing means which is provided with a correcting element that varies power or tension output via gear elements, and which by the build up and withdrawal of the tension or gripping force follow (hysteresis) distinct characteristics. At least one of the measuring elements is disposed in the gripping device for the determination of the respective tensions or forces which are exerted on the workpiece by the gripping elements, whereby the measured values are transferable through a signal controller from the gripping device to the regulator.
2. Description of the Prior Art
Rising centrifugal forces influence the gripping or clamping elements of the rotating gripping devices with increasing rotational speeds, which provide tension or gripping force. In order to also secure a sufficient tension with high rotational speeds, there is already produced a so high tension during the standstill or nonoperational phase of the gripping element, so that there remains a sufficient balance on the gripping power despite the power reduction through the centrifugal force by the high rotational speed operation.
A magnification, such as this, of the tension or force in the standstill of the gripping device has the danger of undesired deformations of the workpiece which is held therein, especially when it concerns a very malleable workpiece, such as a thin-walled ring. With very malleable workpieces such as these, it is therefore frequently necessary to reduce the starting tension in the standstill of the chuck, whereby the danger exists that the tension with high rotational speed operations is lessened by an unduly large degree and leads to a high danger of an accident.
In order to eliminate the disadvantage of the known gripping devices, it has long been known to provide the chuck with counterbalances. Each gripping jaw is associated with the counterbalances over a compound lever arrangement and they adjust the centrifugal force in a known mass. Chucks of this type due to the supplementary counterbalances, therefore are not only complicated in construction, expensive to produce, subject to repair, and make use of, require or demand a comparatively large space, but they also have a different disadvantage, that the fixing equipment and the centrifugal force stress must be adapted one after another. The gripping jaws will be adapted to the respective workpiece by the modern machine tool, not allowing such centrifugal force compensating chucks to be universally used, because in practice a replacement of the counterbalance would be time consuming and expensive.
In order to eliminate this disadvantage, a power operated chuck is known from the German Pat. No. 2,150,885 Steinberger et al dated Sept. 20, 1973, corresponding to U.S. Pat. No. 3,815,929-Steinberger et al dated June 11, 1974. This reference discloses a tension or gripping force dependent upon the centrifugal force which results from the gripping elements of the chuck dependent upon the rotational speed, with this tension being controllable. The control takes place via an impulse transmitter which rotates proportional to the rotational speed of chuck, and which effects a change of pressure by means of a control unit.
The disadvantage of this known construction is that, by the changing of the pressure in the force producing means, neither the lubricating condition of the chuck nor the rigidity of the chuck and the respective workpiece will be taken into account. This limiting condition can influence the actual tension patterns so decidedly, that the amounts which are determined in advance vary considerably from the actual tension patterns, because not the actual tension patterns, but instead only the strength going into the chuck is altered. Due to the advanced improvements of the machining technology, the known construction is no longer adequate.
Moreover, from German Pat. No. 2,557,470 Steinberger et al dated July 7, 1983, a process to distinguish hydraulic cutting of the workpiece in power operated gripping devices is known. In these, the workpiece, which is being gripped, is subjected to a rough machining at a maximum tension and a finishing machining at a lesser tension. In this connection, the tension is produced by means of a pressure medium cylinder, is transmitted by a wedge surface to the tension elements, and, between the rough and the finishing machining, is lowered, without the taking out or unclamping of the workpiece, alone through the reduction of cut pressure in the pressure medium cylinder.
With such gripping tools, with which is transmitted a tension or gripping force, that is produced through a pressure medium activated cylinder, over wedge surfaces to the gripping elements, begins a type of automatic locking through the unavoidable jamming of the transfer elements as a result of deformations of the moved parts in addition to the frictional portion. This occurs whether, the cutting tools which are used in practice with a cutting motion, have a cutting angle which amounts to 15.degree.-20.degree. and is thereby greater than that with an angle of (internal) friction or angle of repose lying at approximately 6. With this lowering of the pressure from a higher pressure for the purpose of the performance of the rough machining to a lower pressure for the purpose of the finishing machining, the cutting strength which corresponds to a high pressure on the workpiece therefore remains operatively in the known chuck with the cutting motion despite the lowering of the pressure.
In order to eliminate this disadvantage, according to the known process, after the conclusion of the rough machining, the cut pressure in the cylinder is controlled under which the automatic locking of the movable gripping elements lowers the maintained value. The reduction of pressure therefore finishes over that for the cohesion of the required values, before joining the desired pressure built up for the finishing machining.
Also with this known process, the overcoming of the hysteresis of the cutting tool cannot be guaranteed despite the controlled lowering of the pressure, since the changes in the cut pressure in the pressure medium cylinder lead to corresponding reactions in the specific cutting tool. Difficulties especially are produced with a disregarding of the lubricating phase of the chuck and the rigidity of the workpiece and gripping device.
These difficulties which arise thereby, that, instead of influencing the cutting strength, the pressure in the tension producing means has been provided as a standard amount- should be eliminated with a power operated chuck according to the published European Patent Application No. 108,857 Rohm dated May 23, 1984. With this chuck, at least one tension receiver is constantly provided on the gripping jaws, i.e. also with a rotating spindle determining the respective cutting strengths, whereby the measured values will transmit over one sender rotating with the chuck and a stationary receiver and will eliminate the requirement to influence the axial strength producing means.
In addition to the constructive features and the possibility to change, with the help of these constructive features, the actual measured cutting strength with the tension receiver, published European Patent Application No. 108,857 Rohm dated May 23, 1984 contains no indication as to how an apparatus for the adjustment of tension is formed, in practice, to attain the desired influence of the tension or cutting strength, especially after the consideration of the foregoing peculiarities with gripping devices, such as friction and hysteresis. Because of these peculiarities, the known regulating systems, which assume a frictionless condition, are not practical.
An object of the present invention therefore is to provide an apparatus for the adjustment of tension on a rotating gripping device of the type previously described, which considers the peculiarities of the gripping elements, especially the occurrence of friction and hysteresis, and not only provides a stabilization of the tension independent of the respective rotational speeds, but also makes possible an alteration of the tension corresponding to the machining program of the machine tool and within a shorter time.